The word "microlens" used in this specification means not only a minute lens whose size is not larger than several millimeters, but also a microlens array formed by one-dimensionally or two-dimensionally aligning a plurality of such minute lenses and a lenticular lens. In this specification, a liquid crystal projector not only means a device having a light source, a liquid crystal display element, image coloring means, an optical system for enlarging and projecting an image displayed by the liquid crystal display element onto a screen and means for driving the liquid crystal display element, but also includes an apparatus in which the above device and the screen are formed as a single piece.
The demand for projection-type liquid crystal display elements such as projection televisions as well as direct-viewing liquid crystal display elements increase. When a liquid crystal display element is used as a projection-type display, if images are enlarged without changing the number of pixels used in a conventional display element, a less definite view will result. In order to obtain highly definite images, it is necessary to increase the number of pixels when enlarging images.
However, if the number of pixels in a liquid crystal display element, particularly, in an active-matrix liquid crystal display element is increased, the area occupied by the pixels becomes relatively small while the area of a black matrix covering other than the pixels increases. If the area of the black matrix increases, the area of the apertures of pixels used for displaying images is decreased and the aperture ratio of the display element is lowered. When the aperture ratio is decreased, the screen becomes darker, resulting in lowered image quality.
In order to prevent a lowering of the aperture ratio due to an increase in the number of pixels, the formation of microlenses on one of the surfaces of a liquid crystal display element was proposed (see Japanese Publication for Unexamined Patent Applications No. 165621-165624/1985 and 262131/1985). The formation of a plurality of microlenses corresponding to the respective pixels enables light which is blocked by the black matrix in a conventional display element to be converged onto a pixel.
In addition, it is possible to use a microlens as: converging means in an optical pick up for laser disks, compact disks and magneto-optical disks; converging means for coupling an optical fiber with a light emitting element or a light receiving element; converging means or imaging means for improving the sensitivity of a one-dimensional image sensor for use in a solid image pickup element such as a CCD and in a facsimile machine (see Japanese Publication for Unexamined Patent Applications No. 17620/1979 and 9180/1982); imaging means for forming on a photoreceptor an image to be printed by a liquid crystal printer or an LED printer (see Japanese Publication for Unexamined Patent Application No. 44624/1988); and a filter for use in optical information processing. Thus, microlenses are used together with various optical elements or optical parts in an optical instrument.
A microlens is manufactured, for example, by the following methods: ion exchange method (Appl. Optics 21(6), p. 1052 (1982), and Electron Lett. 17, p. 452 (1981)); swelling method (Suzuki et al. "New Method for Manufacturing Plastic Microlens", 24th Meeting for Microoptics); "Technique for monolithic fabrication of microlens arrays" (Zoran D. Popovic et al., Appl. Optics 27, p. 1281 (1988)); and machining.
A microlens of distributed refractive indexes is obtained by the ion exchange method. A microlens having semi-spherical refracting surface or paraboloid of revolution (non-spherical refracting surface) is obtained by the other methods. If the microlens is semi-spherical, mass production of the microlens is available by using a semi-spherical microlens as a master (see the 2P method, Japanese Publication for Unexamined Patent Application No. 134103/1993).
By bonding such microlenses on a liquid crystal display element, the effective aperture ratio of the liquid crystal display element is improved, resulting in increased screen luminance. The effective aperture ratio means a transmission rate of a liquid crystal display element without a color filter and a polarizing plate.
However, a liquid crystal display element for use in a projection television, which shows highly definite images with a pixel pitch of around several tens .mu.m, has a reduced aperture area. Thus, there is a limit to improving the effective aperture ratio because the effective aperture ratio depends on a relationship between the size of a spot of light converged by the microlens and the area of the apertures of pixels.
A diameter D of the converged light spot is calculated by EQU D=2.multidot.f.tan .theta. (1)
where .theta. is a divergence half angle of incident light and f is a focal length of the microlens. If the area of the converged light spot becomes larger than the aperture area of pixels, light which does not fall on the pixels is not used for displaying images, thereby limiting the improvement of the effective aperture ratio.
In order to effectively converge light, the divergence .theta. of the incident light and the focal length f of the microlens may be decreased. The divergence .theta. of the incident light becomes smaller as the light emitting area of a light source in use becomes smaller and the distance from the light source to the liquid crystal display element becomes larger. With currently available techniques for light source, however, it is difficult to achieve an angle of less than several degrees for obtaining a longer life and necessary brightness for display. Consequently, there is a need to reduce the focal length f of the microlens and to locate the focal point of the microlens in the vicinity of the aperture of the pixel of the liquid crystal display element.
With the current manufacturing techniques, a liquid crystal display element including pixels with apertures having a side of around 30 .mu.m and a pixel pitch of 50 .mu.m is manufactured. With a liquid crystal display element of this size, if the divergence .theta. of illuminating light is 5.degree., the focal length needs to be set not larger than 170 .mu.m according to the equation (1) in order to achieve a converged light spot with a diameter D of 30 .mu.m. On the other hand, since the convergence of the microlens is proportional to the area thereof, the highest convergence is achieved by arranging microlenses at the same pitch as a pixel pitch P without space, i.e., by setting the microlens diameter to be equal to the pixel pitch P. In this case, the numerical aperture NA of the microlens is NA=P/(2.multidot.f)=0.147. With such a high definition liquid crystal display element in which the pixel pitch P is several tens .mu.m, the numerical aperture of the microlens for reducing the size of a converged light spot is preferably set at least 0.1.
With the structure of the above-mentioned microlens, it is necessary to sandwich glass of a thickness of around 250 .mu.m which corresponds to the focal length of 170 .mu.m in the air (a value obtained by multiplying the refractive index of the glass) so that the focal point is located on the aperture of the pixel of the liquid crystal display element. In order to achieve this structure, a liquid crystal display element may be produced by using a piece of glass with a thickness of 250 .mu.m as a substrate and bonding microlenses thereon. However, this method is not suitable for mass production because such thin glass of a thickness of 250 .mu.m is difficult to handle.
Then, technique for reducing the focal length of a microlens is disclosed in Japanese Publication for Unexamined Patent Application No. 248125/1991. With this technique, cover glass or a cover film of a thickness corresponding to the focal length is attached to a surface of the microlens, and thus the microlens is fabricated within a substrate of a liquid crystal display element. Moreover, Japanese Publication for Unexamined Patent Application No. 233417/1991 discloses a method for achieving mass production and improved adhesion of a liquid crystal display element by forming a lens-like section on a lens substrate by a photosensitive resin according to the 2P method and attaching cover glass having the same coefficient of thermal expansion as the microlens to the lens substrate with a bonding agent having a refractive index different from that of the lens-like section.
However, with the technique for producing a microlens within a substrate of a liquid crystal display element, although there is no need to handle a very thin glass substrate, it is necessary to form transparent electrodes, an alignment film and a black matrix, if necessary, on a substrate (i.e., cover glass) after producing a microlens by attaching the cover glass to the substrate. Thus, this method may cause other problems, for example, a lowering of transparency of the liquid crystal display element due to deterioration of the microlens material and bonding agent, and the separation of the lens from the cover glass. In short, it is hard to say that the productivity in mass production is improved.
More specifically, in a conventional structure, transparent electrodes, an alignment film, a black matrix are formed on a glass substrate under a high temperature not lower than 150.degree. C., generally, around 200.degree. C. Such a heat treatment causes no trouble in the method in which a microlens is bonded to one of substrates after bonding these substrates (i.e., after heat treatment). However, if such a heat treatment is carried out after fabricating a microlens within a substrate, degradation of materials and separation of lens may occur as mentioned above because of the thermal resistance of the microlens material and of the bonding agent.
In order to avoid such problems, the heating temperature may be lowered when forming transparent electrodes, an alignment film and a black matrix on a glass substrate. However, if the heating temperature is lowered, the adhesion of film and the degree of orientation of liquid crystal will be lowered. As a result, the reliability of a liquid crystal display element and of a liquid crystal projector using the liquid crystal display element are lowered, thereby degrading the display quality. Thus, such a method is not suitable.